1. Field of the Invention
The invention relates to optical sensors generally, and more particularly to Fabry-Perot cavity sensors.
2. Discussion of the Background
Optical sensors are used in a wide variety of applications. They offer advantages as compared to other types of sensors, including small size, immunity to electromagnetic interference (EMI), extreme stability, long life, high temperature operation, and low cost. They are especially useful in harsh environments, including high temperature, high pressure environments.
One type of optical sensor is the diaphragm-based Fabry-Perot sensor. In such sensors, a Fabry-Perot cavity is formed between an end of an optical fiber and a reflective diaphragm. Two reflections occur in these sensors: a first reflection between the glass/air interface at the end of the fiber, and a second reflection that occurs at the surface of the diaphragm facing the end of the fiber. If the coherence length of the light source exceeds twice the length of the cavity, observable interference between the two reflections occurs. Deflections of the diaphragm due to a pressure applied to the diaphragm result in changes to the cavity length, which result in corresponding changes in the interference pattern from the two reflections. Some of these sensors are designed such that movement of the diaphragm (and corresponding changes to the cavity length) are constrained to a linear portion of a fringe. This is done to simplify the processing of the signal returned by the sensor.
Diaphragm-based sensors are often formed by attaching an optical fiber to a capillary tube or ferrule (usually glass or silica) and attaching the diaphragm to the tube or ferrule. An example of such a diaphragm based Fabry-Perot sensor is disclosed in U.S. Pat. No. 6,539,135 to Dianov et al. It is typical to use an epoxy to form the attachments between the fiber and ferrule/tube and between the ferrule/tube and the diaphragm in such sensors. However, the use of viscoelastic materials such as epoxies subjects the sensor to time dependent changes, thereby compromising the reproducibility and operation of the sensor. In addition, the use of viscoelastic materials increases the temperature dependence of the sensor.
PCT Publication No. WO 99/60341 discloses diaphragm-based Fabry-Perot sensors formed by a fiber surrounded by a ferrule/tube and a silicon wafer with a portion etched away to form a Fabry-Perot cavity. Several different methods for attaching the components of the sensors are disclosed. WO 99/60341 discloses bonding a fiber to a capillary tube using epoxy. In order to minimize thermal drifts, WO 99/60341 discloses attaching the fiber to the front end of the ferrule/tube locally by heating the capillary with a laser or local heating element and allowing the capillary to collapse along a limited section of up to a few millimeters of the fiber. In such embodiments, a flexible adhesive is used to bond the fiber to the ferrule/tube to allow for movement to alleviate stresses from thermal mismatches between the fiber and tube/ferrule. Applicants have experimented with such a procedure but the mechanical bond between the collapsed portion of the capillary tube and the fiber that results from this process has proven unsatisfactory. WO 99/60341 also discloses using solder glass to adhere the fiber to the ferrule/tube, but does not explain how thermal mismatches between the tube/ferrule and the fiber are accommodated. With respect to the bonding of the diaphragm to the ferrule/tube, WO 99/60341 discloses using adhesives, anodic bonding and diffusion bonding. The techniques disclosed in WO 99/60341 are an improvement over the use of epoxies, but are not ideal.